General:
* Unpredictable
* Inconsistent
* Unreliable
* Capricious
* Whimsical
* Irregular
* Fluctuating
* Variable
* Unstable
More specific:
* Volatile (for sudden and dramatic changes)
* Fickle (for changing opinions or moods)
* Mercurial (for quick and unpredictable changes in mood)
* Rambling (for a lack of focus or coherence)
* Sporadic (for happening irregularly or randomly)
The best synonym depends on the specific context in which you're using "erratic."